1. Field of the Invention
The present invention relates to a stain-proofing agent to be used for stain-proofing treatment of the surfaces of, for example, wood fiber cement boards, calcium silicate boards, cement (concrete) boards, metal plates or boards, or glass plates or boards, as well as to a building board whose surface is treated with the stain-proofing agent.
2. Description of the Prior Art
Building boards such as, for example, external wall materials are generally coated with a coating composition on their surfaces. It has been proposed to apply thereto a stain-proofing agent which forms a stain-proofing film having a self-cleaning function to remove stains adhered to the surfaces after attachment.
As this kind of stain-proofing agents has been used such an agent which forms a super hydrophilic stain-proofing film on the surface to be treated. Upon application of the stain-proofing agent onto the surface of a substrate, a super hydrophilic stain-proofing film is formed thereon. When stains are adhered to the surface of the substrate, water applied to the surface is absorbed by the super hydrophilic stain-proofing film and, as a result, the stains float on the water and are washed away together with the water (i.e. self-cleaning effect).
In order to form a super hydrophilic stain-proofing film on the surface of a substrate, a stain-proofing agent mainly consisting of an aqueous dispersion of silica fine particles (colloidal silica) has been hitherto used.
For example, Japanese Laid-open Patent Publication No. 6-71219 gazette (JP 6-71219 A) discloses a method for forming a stain-proofing film which comprises applying an aqueous dispersion of colloidal silica having an average particle diameter of not more than 100 nm to a coat formed from an aqueous emulsion of a synthetic resin to form a film of colloidal silica on the surface of the coat.
Japanese Laid-open Patent Publication No. 2002-338943 gazette (JP 2002-338943 A) discloses a method for forming a stain-proofing layer which comprises applying a liquid containing colloidal silica and an alumina/aluminum-magnesium composite oxide to a coated surface for providing the coated surface with water-proof and alkali-proof properties.
The above-described silica fine particles give super hydrophilicity to the treated surface of a substrate owing to the presence of silanol groups on the surface of the particles.
The above-described silica fine particles contain a number of vicinal silanol groups in which silanol groups present on the surface of the particles are adjacent closely to one another. Since the vicinal silanol groups are mutually hydrogen-bonded, the concentration of free silanol group (i.e. single silanol group) which participates in hydrophilicity is not so high. Thus, in order to obtain a stain-proofing film having a high hydrophilicity, it is necessary to increase the concentration of silica fine particles in the aqueous dispersion.
However, a high concentration of silica particles is disadvantageous in that the resulting aqueous dispersion becomes expensive and uneven application of the aqueous dispersion results in a whitish pool of silica fine particles, whereby providing a color different from the inherent color of the coat.
Furthermore, it is necessary that the above-described stain-proofing film has a good adhesion to the coat of a coating composition formed on the surface of a building board.
If the adhesion between the stain-proofing film and the coat is low, the stain-proofing film is peeled off at an early stage to fail to provide a long-term effect and durable stain-proofing effect.
In an attempt to enhance the adhesion between a stain-proofing film and a coat, there have been proposed a method for oxidizing the coat by means of flame or an oxidizing agent (e.g. Japanese Laid-open Patent Publication No. 2002-336768 gazette), and a method for applying a stain-proofing agent after the surface of the coat is heated to 60° C. or more (e.g. Japanese Laid-open Patent Publication No. 2002-330769 gazette).
However, if a coat on the surface of a building board which is required to have a molded surface and a delicate texture is treated with flame or an oxidizing agent, the coat may deteriorate to discolor and lose glaze, whereby reducing the commercial value of the building board, and generation of peeling off, cracks, and pinholes may cause deterioration in the performance of the board.
In addition, heating of the surface of the coat to 60° C. or more is difficult in terms of temperature control, may cause uneven treatment, and is not favorable in view of energy saving.